If multiple streams of chemical reactants are combined with less than optimum mixing, less than acceptable output product compositions may result. Examples of such compromised output products include non-uniform seed precipitation products, incompletely reacted source materials, isolation of unreacted source material by reacted product, liquid striation, and the like. Mixing effects are particularly significant in high reactivity or low diffusivity systems where concentration inhomogeneities cannot be leveled off before appreciable reaction occurs. Some reactions in low viscosity systems are fast enough, relative to conventional mixing, that conversion occurs in a region or reaction zone where partial segregation of the reactants causes steep concentration gradients which can result in a reduced reaction rate relative to perfect mixing.
A stirred tank, e.g. an autoclave, is the most widely used industrial chemical reactor type, accounting for about 50% of the world's chemical production in terms of value. However, a stirred tank reactor has several limitations, especially when used to perform fast chemical reactions. The flow field is inhomogeneous and generally of low intensity and the predominant flow pattern is back-mixing, making scale-up difficult due to the complexity of the flow. This often leads to large scaling ratios, i.e. due to inefficiencies in mixing more source materials are required to obtain a desired output quantity.
Accordingly, there is a need for a chemical mixer/reactor which can overcome these and related problems, particularly for continuous reactions which entail the precipitation of sparingly soluble materials from two or more liquid ionic solutions and subsequent crystallization, i.e. nucleation and growth of crystals, of the reaction product. With conventional mixing the resulting solid product usually has a wide crystal size distribution which determines the filtration, washing, settling abilities of suspensions, and thus the quality, e.g. crystal size homogeneity, surface area, and the like, of the resulting product.
With the present invention, control and optimization of the macromixing, mesomixing, and micromixing parameters for a particular reaction allows preparation of uniform output products with consistent, uniform quality. "Macromixing" refers to the general distribution of an additive or reactant on a coarse scale in a mixer via turbulent dispersion. In the continuous system of this invention, global axial macromixing can be characterized by a residence time distribution and global radial macromixing by a coefficient of variation. "Mesomixing" refers to mixing at an intermediate scale between macromixing and micromixing. Mesomixing results in a reduction of the scale of segregation between reactants, i.e. feed eddies are reduced in size to small "engulfment" sized eddies. "Micromixing" refers to the final stages of mixing (engulfment in small eddies and molecular diffusion) which brings together liquid reactants on a molecular scale.
It is also known that mixing can dramatically affect the properties of the final product of a chemical reaction, particularly a precipitation reaction. As a result, U.S. Pat. Nos. 5,417,956 and 5,466,646 disclose methods of producing nanosize particles by use of the emulsion forming apparatus of U.S. Pat. Nos. 4,533,254 and 4,908,154. The process entails premixing two liquids which react with each other and then subjecting that single premixture to high pressure and a subsequent high energy mixing process, i.e. high shear, preferably using a Microfluidizerw device, to generate nanosize particles. It has now been recognized that for a precipitation reaction, the reaction kinetics are such that significant precipitate nucleation and reaction will have occurred during the premixing, well prior to introduction of the mixture into the high energy mixing zone. As a result, the nuclei formed in the premixture are less uniform than when the mixing/reacting is performed with the apparatus of the present invention. It is now believed that the predominant mode of operation of the Microfluidizer.RTM. device is breaking up precipitant material to produce smaller particles, rather than direct control of the size of the precipitant material during its initial formation. The present invention produces nanosize product particles of a smaller, more uniform size distribution that enhances use of the products for various applications, including in the field of catalysis.
In a Microfluidizer.RTM. device a single hydraulically-driven intensifier is moved in an axial reciprocating stroke to pressurize and process a single stream of pumpable fluid. No matching of flows of two or more different fluid streams, nor use of separate high pressurizing pumps for each different fluid stream, nor flow control of multiple streams are required.
Control of the flow of two or more pumpable fluid source materials, both individually and in relation to each other, at high pressure is critical to the present invention. At low pressures, up to a maximum of about 5,000 psi, as used in hydraulically-driven systems, flow control is obtainable with proportional control valves. However, at the reactant stream pressures used in the present invention, i.e. about 8,000 psi and above, control valves and materials are not available to provide the required degree of flow control for the low viscosity fluids used herein. While gear pumps have been used to extrude high viscosity thermoplastic materials at pressures up to 30,000 psi, they are not useful with the low viscosity source material streams of the present invention. The low viscosity streams would push past the gear-to-housing seals and the gear pumps could not provide adequate flow regulation. Moreover, even making flow measurements at pressures above 5,000 psi exceeds the capability of many conventional fluid flow techniques. Moving part techniques such as rotating vane pick-ups, mass flow meters such as hot wire dissipation, transient measurement techniques such as orifice flow meters, and flow meters using ultrasonic transducers all encounter serious technologic barriers in the high pressure environment of this invention.
It is an object of the present invention to create a continuous mixer/reactor wherein macromixing, mesomixing, and micromixing parameters can be varied and controlled so that the output product will have intended characteristics and be of uniform quality.
It is a further object of the present invention to create a continuous mixer/reactor wherein the proportions and weights of two or more source materials at high pressure are monitored and controlled.
It is a further object of the present invention to create a continuous chemical mixer/reactor which will provide increased control over the nucleation stage of a precipitation reaction as compared to conventional reactors so as to enhance overall product quality by directly producing products having generally one or more of smaller crystallite sizes (without the need for any grinding or milling operation), a more uniform chemical composition, a narrower distribution of crystallite or particle sizes, and/or previously unobserved phases and crystallite morphologies.
It is a still further object to create a chemical mixer/reactor which can allow reactants to react sufficiently rapidly to enhance the selectivity of the reaction to generate more desirable products as opposed to less desirable ones.
Other objects of the invention will be evident from the ensuing detailed description of this invention.